linux.c: Detect hall-of-mirrors effect in `nf_conntrack'.

linux.c: Detect hall-of-mirrors effect in `nf_conntrack'.

As well as NAT entries, this file contains entries for ordinary
connections, where both address pairs are equal and swapped.  If we find
one of these -- because we failed to find the connection in the main
`tcp' table for some reason -- then we'll pick the other remote address
as an ident daemon to talk to.  The problem is that that other daemon is
us, and we'll end up talking to another instance of ourselves, which
will fall into the same trap.  Until there aren't any file descriptors
left.